Thank you all, guys, you're amazing as always.

So, I very much suspect temp is an issue here. Every failed drive I pulled out of the box was nearly "OMFG hot". I say "nearly" just because occasionally I've touched hotter drives, but yes, these are on the very hot end of the spectrum of my personal experience. So, yes, great point, cooling is not ideal for sure.
Also, I started this thread because one drive just failed on me, yesterday, and interestingly it happened when temperature in Rome rised significantly and, in spite of AC at home, temp in the room where the drobo also increased.

Also, as you are correctly noticing, I've also been wondering if Drobo is being too conservative here and reports a drive as Failed when a PC or other NAS models would probably not. Only once I've tried to connect a drive to a PC (I should've done that more often), and interestingly it was recognized and I could format it. But, it became unresponsive the day after, as I tried to stress it a bit by running a backup script TO it. So, I decided it was dead and did not spend more time analyzing.
However, latest failed drive is till at home on my desk, I'll give it a try and see if it is in fact still alive.

I suspect there's a combination of factors here, maybe:

1. Poor cooling
2. Possibly, conservative approach at marking a drive as "Failed"
3. Old drivers being used in my particular case.